Also, the Grand Comics Database, which is “a nonprofit, internet-based organization of international volunteers dedicated to building an open database covering all printed comics throughout the world”

Marvel and DC both have their own dedicated wiki pages for their respective characters.

For places to download (free and legally) old comics, there’s the Comic Book Archive, The Digital Comic Museum, and My Comic Book Plus. These are so much fun to browse through and see what comic books from the 30s, 40s and 50s looked like. Some of these scans even include the original ads, which is always a fun bonus (and great research sources).

And just for fun, if you’re looking for comic strips instead of books, Go Comics has a great clearning house.